1. Field of the Invention
The present invention relates to a magnetron for use in a microwave using apparatus such as a microwave oven.
2. Description of the Related Art
Conventionally, there is proposed a technology which uses a getter for the purpose of enhancing the degree of vacuum in the inside of a magnetron (for example, see the patent reference 1). FIG. 5 is a longitudinal section view of a magnetron for use in a conventionally general microwave oven. In FIG. 5, in the inside of a cylindrical-shaped anode barrel member 10, there are radially arranged anode vanes 11; and, there are provided cavity resonators formed of spaces respectively enclosed by the mutually adjoining anode vanes 11 and anode barrel member 10. In the central portion of the anode barrel member 10, there is provided a cathode structure member 12; and, a space enclosed by the cathode structure body 12 and anode vanes 11 provides an action space 19.
The cathode structure member 12, as shown in FIG. 6 which is a partial section view of the cathode structure member 12, comprises: a filament coil 121 made of thorium tungsten; an upper end hat 122 and a lower end hat 123 respectively made of molybdenum for supporting the two end portions of the filament coil 121; a center lead 124 having a leading end portion fixed to the upper end hat 122 and penetrating through the lower end hat 123 in such a manner that it is not in contact with the filament coil 121; and, a side lead 125 the leading end of which is fixedly secured to the lower end hat 123. The filament coil 121 is fixed to the upper and lower end hats 122 and 123 by high frequency brazing. As the material of the filament coil 121, there is used the above-mentioned thorium tungsten and, in order to increase the quantity of emission of electrons, on the surface of the filament coil 121, there is provided a carbonized layer by applying a current to the filament coil 121 in a hydrocarbon system gas to thereby heat the filament coil 121. The current, which heats the filament coil 121, flows in order of the center lead 124, upper end hat 122, filament coil 121, lower end hat 123 and side lead 125 or in reverse order.
On the top surface of the upper end hat 122, there is disposed a getter 126 which is used to enhance the degree of vacuum of the inside of the magnetron. As known well, when a magnetron is operated, there is emitted a gas from the composing members of the magnetron and, due to the gas, the degree of vacuum of the inside of the magnetron is lowered and the oscillation efficiency of the magnetron is thereby lowered, which can raise a fear that the oscillation of the magnetron is caused stop. To solve this problem, by disposing the getter 126 made of titanium, zirconium or the like in the inside of the magnetron, the gas emitted from the composing members of the magnetron is absorbed to thereby prevent the lowered degree of vacuum.
By the way, the getter 126 may also be disposed on the lower end hat 123 instead of the upper end hat 122. Also, in the magnetron disclosed in the patent reference 1, the particle diameter of gas absorbing metal powder used as the getter 126 is set for 10 μm or smaller to thereby not only prevent the getter 126 from peeling off from the top surface of the upper end hat 122 but also enhance the getter effect.
Referring back to FIG. 5, to the upper end of the anode barrel member 10, there is fixed a pole piece 14; and, to the lower end thereof, there is fixed a pole piece 15. The pole pieces 14 and 15 are respectively formed in a funnel-like shape by drawing a plate member made of magnetic material having small magnetic resistance such as iron. In the pole piece 14, there is opened up a hole through which an antenna 16 can be passed. Just above the pole piece 14 and just below the pole piece 15, there are closely mounted ring-shaped magnets 17 each of which has a hollow central portion, respectively. Through the magnet 17 mounted just above the pole piece 14, there can be penetrated the antenna 16. As the magnet 17, from the viewpoint of reducing the size of the whole of the magnetron and making the magnetron easy to handle, there is used a ring-shaped permanent magnet using ferrite; and, one end portion of the magnet 17 is closely contacted with the pole piece 14(15). A yoke 18 is used to magnetically connect together the other end sides of the magnets 17 and the pole pieces 14 and 15, and the yoke 18 is made of a plate member having small magnetic resistance such as iron. That is, the upper and lower magnets 17 are respectively connected to the pole pieces 14 and 15 magnetically by the yoke 18.
The anode barrel member 10, together with the anode vanes 11 respectively formed in the inside thereof, is made of material such as oxygen-free copper which can radiate heat well and is hard to generate gas. The reason for this is that, when the following two facts are taken into consideration, a material which can provide good electric conduction and heat conduction is preferred: that is, one fact is that the material is heated by impacts generated when electrons fly into the leading end portions of the anode vanes 11; and, the other is that, when the anode vanes 11 and anode barrel member 10 cooperate together to form cavity resonators and, within the cavity resonators, microwaves are resonated and oscillated, a large amount of high frequency currents flow in the respective surfaces of the anode vanes 11 and anode barrel member 10.
When the conventional magnetron is used, the inside of the anode barrel member 10 is evacuated and a direct current high voltage is applied to and between the anode vanes 11 and cathode structure member 12. In the action space 19, there is formed a magnetic field due to two magnets 17. As the direct current high voltage is applied to and between the anode vanes 11 and cathode structure member 12, thermoelectrons discharged from the cathode structure member 12 fly out toward the anode vanes 11. At the then time, the magnetic field generated by the two magnets 17 concentrates in a gap formed between the pole pieces 14 and 15 and thus, in the action space 19, the magnetic field acts in a direction perpendicular to a direction where the cathode structure member 12 and anode barrel member 10 are opposed to each other. As a result of this, while the thermoelectrons discharged from the cathode structure member 12 are caused to circle due to a Lorentz force received from the magnetic field caused by the two magnets 17, they turn around the periphery of the cathode structure member 12 and then arrive at the anode vanes 11. Energy generated due to the then time electron motion is applied to the cavity resonators, which contributes to the oscillation of the magnetron.
Patent Reference: JP-A-2004-281320
By the way, in the case of a magnetron, since it discharges electrons in the inside thereof, when the quantity of electrons to be discharged is large, there increases noise. As a method for reducing the noise, there are available a method which reduces the quantity of input power (that is, which reduces the quantity of a current flowing in a filament coil to thereby lower the temperature of the filament coil and thus restrict the quantity of thermoelectrons to be discharged), and a method which changes the line diameter or pitch of a filament coil 121 to thereby reduce the electron discharge area of the filament coil 121. However, in both of these methods, a getter effect (that is, a gas sucking effect) cannot be displayed sufficiently. In the conventional magnetron shown in FIGS. 5 and 6, a getter 126 is heated by heat discharged from the filament coil 121, whereby the getter effect can be displayed; however, when the quantity of the input power is reduced, it seems that the quantity of heat radiated from the filament coil 121 is reduced and thus the heating of the getter 126 becomes insufficient, which results in the lowered getter effect.